Adaptive Coding and Modulation (ACM) is an effective technique for providing link adaptation in both Uplink (UL) and Downlink (DL) communications. ACM is typically accomplished by algorithms working together in both the receiver and the transmitter of a WTRU and/or a base station. The receiver makes an estimate of the channel quality by measuring the Signal-To-Interference Ratio (SIR) of one or more transmissions from the transmitter.
Each CQI may correspond to a particular configuration of radio resources such as code rate and modulation type. After each SIR measurement is made, the CQI is computed. For example, the SIR may be compared to a table of SIR-CQI pairs and the CQI value that yields the best performance, for example in terms of Block Error Rate (BLER) or throughput, is selected. This is typically performed at the receiver and sent back to the transmitter. The transmitter then selects a radio configuration that is no more aggressive than that indicated by the received CQI value. In an otherwise unpopulated cell, the transmitter would simply use a configuration consistent with a channel quality indicated by the CQI.
Although maintaining the appropriate link adaptation parameters, (e.g. code rate, modulation type, number of codes and power control), is known to optimize throughput for the radio link, there are cases where the parameters selected by the transmitter will be out-of-sync with channel quality. In essence, the parameter selected by the transmitter will not correspond to the CQI judged to optimize the throughput. This case can arise when for example there is insufficient data available for the next transmitted packet, the scheduler decides to share resources that would otherwise be required, or during conformance testing.
Another problem is that channel quality estimates may be more heavily quantized than the available set of radio resources, thus the assumed one-to-one mapping between CQI and radio resource configuration is lost. This is particularly true in Third Generation Partnership Project (3GPP) system.
The problem can be illustrated by considering the behavior of an adaptive bias algorithm when the transmitter occasionally uses more conservative configurations than indicated by the CQI report from the receiver. When the more conservative configurations are used, errors will be less frequent and the adaptive algorithm will adapt to request more aggressive configuration of resources to maintain the target BLER. When the transmitter returns to normal operation, responding as anticipated to CQI reports, the bias will have been adapted to the wrong value and many packets will be lost until reconvergence of the algorithm.